1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular, an image forming apparatus to which sheet containing means containing sheets is detachably attached.
2. Related Background Art
A conventional image forming apparatus allows sheet containing means containing sheets to be detachably attached to the main body of the image forming apparatus. In such an image forming apparatus, sheets contained in a sheet containing cassette serving as the sheet containing means are successively conveyed to an image forming portion, where image recording is effected on the sheets.
Incidentally, the sheet containing cassette is attached and detached for the purpose, for example, of filling it with sheets or effecting a change in sheet size. When the attachment operation is conducted on the sheet containing cassette after filling it with sheets, the sheet containing cassette, which has become heavy due to the sheets, puts a significant burden upon the user. Further, when the sheet containing cassette is heavy, the user may exert an excessive force at the time of attachment to give the cassette great momentum in the attachment direction thereof, resulting in an impact at the time of attachment. This impact may lead to positional deviation of the sheets filling the sheet containing cassette or damage to the sheet containing cassette.
As means for solving this problem, there is known, for example, an automatic sheet containing cassette conveying mechanism, which, when the insertion of the sheet containing cassette is detected, automatically conveys the sheet containing cassette by the drawing portion, thereby sparing the user the trouble of performing the operation of inserting the sheet containing cassette. In the case of this mechanism, however, automatic conveyance is impossible in a state in which no power is supplied to the apparatus main body. In view of this, there is provided charge means for storing driving power, making it possible to perform automatic conveyance even when no power is being supplied. The provision of such charge means, however, results in a very complicated apparatus construction.
In view of this, Japanese Patent Application Laid-open No. H4-85220, for example, discloses, instead of an automatic sheet containing cassette conveying mechanism using electric power, a mechanical automatic sheet containing cassette conveying mechanism using a spring and needing no electric power. As shown in FIGS. 13A and 13B, in such a mechanical automatic sheet containing cassette conveying mechanism, the conveyance of the sheet containing cassette is performed as follows: by using a toggle spring 120, a rotation moment is generated in an arm 118 rotatable around a shaft 119; the action force of the rotation moment is transmitted to an engagement pin 113 provided on the back surface of the sheet containing cassette; and the engagement pin 113 is drawn along a guide 123 extending linearly in the direction in which the sheet containing cassette is inserted, thereby drawing the sheet containing cassette.
However, in this conventional image forming apparatus adopting a mechanical automatic sheet containing cassette conveying mechanism, when drawing the sheet containing cassette through the engagement pin 113, the arm 118 makes a rotating motion, whereas the movement route for the engagement pin 113 formed by a groove 124 defined by the guide 123 extends linearly in the direction in which the sheet containing cassette is inserted, so that the engagement pin 113 is guided linearly.
Here, when the engagement pin 113 is thus guided linearly, the position of the point of action of the moment on the arm transmitting force to the engagement pin 113 changes every moment as the sheet containing cassette is drawn, and in the end region of the sheet cassette conveyance section, the point of action of the moment with respect to the engagement pin 113 moves away from the shaft 119, the point of action of the moment having been moving toward the shaft 119 of the arm 118 due to the configuration of a grasping groove 118a for grasping the engagement pin 113, which is formed at the rotation end of the arm 118.
Further, when the point of action of the moment thus moves away from the shaft 119, which is the rotation center of the arm 118, the conveyance force for drawing the sheet containing cassette is weakened as indicated by symbol a in FIG. 7 (described later). Furthermore, a shortage of conveyance force occurs in the portion where the conveyance resistance to the sheet containing cassette increases due to a spring component, etc. of a detecting sensor or the like for detecting the accommodation of the sheet containing cassette by utilizing a spring. As a result, it is impossible to draw the sheet containing cassette reliably to a desired position.
It might be possible to prevent this shortage of conveyance force by using a strong toggle spring. In this case, however, the requisite operating force for pulling back the toggle spring when drawing out the sheet containing cassette increases, resulting in a deterioration in usability.